As shown in FIGS. 4 and 5, a charging apparatus 100 so-called corotton, an example of a conventionally used charging apparatus in a copying machine, a printer or the like comprises insulating fixing member end blocks 102 installed at both ends of the metal shield member 101 and a metal discharge wire 103 supported between the end blocks 102 with a specified tension. An end of the discharge wire is fixed to one end block 102 by a screw 104, and the other end of the wire having a spring 105 is fixed to the other 102 by another screw 104. The shield member 101, as shown in FIG. 5, is disposed so as to surround the discharge wire 103 except where the wire faces a photoreceptor drum 106 of a copying machine or the like. The distance between the walls of the shield member 101 and the discharge wire 103 and the distance between the discharge wire 103 and the photoreceptor drum 106 are arranged to be approximately in the range from 8 to 15 mm. The discharge wire 103 is a wire of tungsten having a diameter of 30 to 100 .mu.m. Aluminum, stainless steel or metal plated steel plate is used for the material of the shield member 101, and synthetic resin is used for the end blocks 102.
in the thus structured charging apparatus 100, as shown in FIG. 5, a voltage which is at least equal to a corona discharge starting voltage (usually some kV) whose level is mainly determined by the diameter of the discharge wire 103 and a distance between the shield member 101 is applied to the discharge wire 103 by a power supply 107, which forms a large electric field around the surface of the discharge wire 103 as shown in FIG. 6, and partial dielectric breakdown, namely corona discharge is produced inside the electric field R. According to the principle of corona discharge, stable discharge is maintained in the vicinity of the wire 103. In the charging apparatus 100, uniform charging on the surface of the photoreceptor drum 106 is carried out by moving ions produced by the corona discharge to the surface of the photoreceptor drum 106 through the electric field formed between the discharge wire 103 and the photoreceptor drum 106 and attaching the ions to the surface of the photoreceptor drum 106.
Documents such as "Denshishashin ni okeru coronahouden" (Corona Discharge in Electrophotography) N. Kutsuwada, Seidenkigakkai-shi, Vol. 12/6, 1988, pp. 409-412 and "Denshishashin no coronahoudensouchi no kadai (Topics in Corona Discharge for Electrophotography" N. Yamazaki, Seidenkigakkai-shi, Vol. 12/6, 1988, pp. 418-425, however, describe various problems in the conventional art, because a conventional charging apparatus 100 is structured so as to produce ions, using corona discharge, to charge the photoreceptor drum 106 of the copying machine or the like.
First, if the charging apparatus 100 using corona discharge is operated for a long time, discharge products which are insulating solids mainly comprising SiO.sub.2 form on the discharge wire 103, which prevents the surface from producing a corona discharge easily. It lowers the corona current and leads to a decline of copy image density and non-uniformity.
Second, corona discharge of the charging apparatus 100 produces ozone, which not only impairs the quality of the photoreceptive member but also, if the amount is significant, is harmful to humans. Therefore, it is necessary to, provide an ozone filter in a copying machine to prevent the leakage of ozone.